The present invention relates to laser direct image printing and more specifically to a method and apparatus including a mode locked laser utilizing scophony methods to increase image quality and therefore line sharpness and accuracy.
It is known today that printed circuit boards may be composed of several PCB panels, each panel having two sides, one or more of which is provided with a layer forming an electrical circuit. When there is only one panel having only two layers, the board is commonly called a double-sided board or PCB panel, and when there are more than two layers, the board is commonly called a multi-layer board. A common way of manufacturing a multi-layer board is by fixing several panels together, each panel having a single printed circuit on one side, or a circuit on each side. xe2x80x9cOuterxe2x80x9d panels are those that face the outside of a multi-layer PCB, and xe2x80x9cinner panelsxe2x80x9d are the interior panels. Typically, the inner panels have a circuit on both sides, while the outer panels have a circuit only on one, the outer side. Each inner panel resembles a thin double-sided PCB in that the panel is comprised of an insulating substrate, which is clad on both sides with metallic foil, typically copper foil. A printed circuit is formed on any circuit side of an inner panel by that side""s metal cladding having a light-sensitive layer laid on top of the metal. The light-sensitive layer is exposed to light (typically ultra-violet (UV) radiation) at selected locations, then processed by a photographic process that removes the layer at selected locations. A metal etching process is then applied to remove those parts of the layer of metal not necessary for forming the actual circuit. Once all the double-sided inner panels are produced, they are fused (pressed) together by placing an insulating binding material, typically a partially cured epoxy-resin material called prepreg, between the panels. Unexposed outer foils are placed on the outside of the double-sided inner panels, again with prepreg in between. All the layers are now laminated by applying heat and pressure that causes the prepreg to flow and bond to the surfaces of the inner panels and the outer foils. Holes are now drilled on the laminated multi-layer board, including holes for mounting electrical components inserted into the board (xe2x80x9cmounting holesxe2x80x9d), and holes for making contacts from one layer to one or more other layers (feed-throughs, also called vias or conductive vias). The holes typically are plated through. Each outer side of the multi-layer panel now is sensitized, then exposed and processed to form the two outer printed circuits in exactly the same manner as forming circuits on the inner panels.
Since a multi-layer panel is exposed in the same way as an inner PCB panel, xe2x80x9cPCB panelxe2x80x9d or simply xe2x80x9cpanelxe2x80x9d means either a complete PCB board, an inner PCB panel, or a post-lamination multi-layer panel.
A common method for producing printed circuit boards is to first produce artwork, which is an accurately scaled configuration used to produce a master pattern of a printed circuit, and is generally prepared at an enlarged scale using various width tapes and special shapes to represent conductors. The items of artwork, once reduced, for example, by a camera onto film to the correct final size, are referred to as phototools and are used as masks for exposing the sensitized layers. Because the photographic reduction is never 100 percent accurate, more accurate phototools are produced nowadays using photoplotters rather than photographic reduction.
However produced, physical phototools are susceptible to damage. In addition, whenever any amendments need to be made to any circuit, new phototools need to be produced. Furthermore phototools, sometimes in the form of photographic negatives, are difficult to store. They also may not be stable; their characteristics might change with temperature and humidity changes and can suffer degraded quality over time.
Many of the disadvantages of using phototools can be overcome by using direct imaging technology, for example with a laser direct imaging (LDI) device. The working and benefits of such LDI devices are known. LDI may be performed by scanning a laser across the surface of a PCB panel from one edge of the PCB panel to the other edge, along one or more scan lines. For examples of LDI systems and their use, see U.S. Pat. No. 5,895,581 to Grunwald (issued Apr. 20, 1999) entitled LASER IMAGING OF PRINTED CIRCUIT PATTERNS WITHOUT USING PHOTOTOOLS, and U.S. Pat. No. 5,328,811 to Brestel (issued Jul. 12, 1994) entitled METHOD OF PRINTING AN IMAGE ON A SUBSTRATE PARTICULARLY USEFUL FOR PRODUCING PRINTED CIRCUIT BOARDS. See also co-pending U.S. patent application Ser. No. 09/435,983 to Vernackt, et al. (filed: Nov. 8, 1999), entitled: METHOD AND DEVICE FOR EXPOSING BOTH SIDES OF A SHEET, assigned to the assignee of the present invention and incorporated herein by reference for all purposes.
One difficulty in producing multi-layered printed circuit boards is the strict requirement for accuracy in positioning the different PCB panels together to ensure that the different circuits are positioned very accurately relative to each other. In particular, the mounting holes and vias need to be very accurately placed on each layer""s circuits. For a particular tolerance for the placement of a circuit, it is clear that any deviations in the specified location of the circuits on each of the layers may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including the multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
Registration is the process of positioning the PCB pattern on the panel at a particular physical location. Thus, in the case of direct laser imaging, it is where the panel is physically positioned relative to the laser beam.
The relationship between imaging process and the registration process becomes increasingly important when higher geometrical accuracy higher PCB layout density are desired.
The geometrical accuracy can be increased by the use of a laser direct imaging (LDI) device. However, to achieve such benefits, both geometrical accuracy and the quality of imaging are important. In particular, the repeatability, line edge quality and control of the line width of the tracks after etching (i.e., the widths of the conducting interconnects) are important. Further, more and more circuit components such as coils, high frequency (HF) circuits, and oscillator circuits are nowadays being implemented within the PCB layout itself. It is necessary to predict the characteristics of those components, and for this, a known and controlled fabricating process is needed to substantially eliminate later circuit trimming. LDI technology addresses some of these problems and increase the overall imaging quality.
New technology for making PCB panels like sequential build up (SBU) and direct ablation of the copper can be used with direct imaging technology. Accuracy is also important for such new technologies that include adding each new layer directly to the previous stack of layers as an additive process. In such a case, the relationship between the imaging process and the registration process becomes very critical.
FIG. 1 illustrates one method of producing the PCB 200 illustrated in FIGS. 2A-2E. In block 102, a substrate 202 with a copper layer 203 is provided. Next, a layer of photoresist 204 is applied on top of the copper layer 203, in block 104. Then a mask layer 206 is placed on top oil the photoresist 204 in block 106. The mask layer has at least one opening 208 substantially corresponding to the location, shape and size of the desired copper trace 220. In block 108 the photoresist 204 that is not covered by the mask layer 206 is exposed with a high intensity light 210 such as an UV lamp. Next, in block 110, the mask layer 206 and the unexposed photoresist 204 is etched away. Then, the exposed portion of the copper layer 203 is etched away in block 112. The exposed photoresist 218 is then removed in block 114. In block 116, only the desired copper trace 220 remains.
When a direct imaging technology is used, step 106 is not used, and the mask layer 206 is not required. Step 108 is then replaced with a direct imaging step that exposes some areas of the photoresist and not others in accordance with imaging data that corresponds to the pattern desired.
As illustrated in FIGS. 2D and 2E, the exposed region of the photoresist 218 is typically wider at the bottom 224 than at the top 222, i.e., the sides of the exposed photoresist 218 are not perpendicular to the substrate 202. The top 222 is substantially the same width as the opening in the mask 208. The resulting copper trace 220 is similarly wider than the opening in the mask 208 (FIG. 2A). The triangular areas 214 and 216 represent an inaccuracy of the process 100. These areas 214, 216, while illustrated as having a triangular cross-section, are typically irregularly shaped, as known to those skilled in the art. FIG. 3 illustrates a PCB substrate 302 with an irregular trace 304 resulting from such inaccuracy, together with the opening in the mask 306 that corresponds to the desired trace.
As shown in FIG. 2E, the areas 214, 216 (shown triangular) are typically larger when non-collimated light source is used than when a collimated beam is used. If a perfectly collimated exposing light source is used, no error should, in theory, occur, assuming no other processes produce inaccuracies. A laser direct imaging (LDI) device approaches a near perfectly collimated light source but, as will be explained below, still does not eliminate the error-producing areas 214, 216.
The walls of any exposed resist area are called sidewalls herein. Sidewall quality degradation contributes to line edge quality degradation. Other causes of sidewall quality degradation in addition to non-perfect collimation are present when phototools (and other masks) are used. One reason is that the light must travel through a certain transparent layer 205, causing the light to be at least slightly diffracted by the diffraction coefficient of the transparent layer 205. Since LDI devices do not use such phototools or a transparent layer, this diffraction error is eliminated and thus inherently increases the quality.
There thus are advantages to directly imaging the required circuit patterns onto PCB panels, for example PCB panels that include a light-sensitive layer on one or both sides. Directly imaging PCB panels in particular improves geometrical position, provides collimated exposure, and eliminates diffraction-related errors. Note that the same advantages also are provided when directly imaging printing plates that include a UV, visible light, or thermally-sensitive layer.
Often such sensitive sheets as used for PCBs or thermal printing plates are rigid, so that the scanning apparatus for exposing such sheets for direct imaging (e.g., directly exposing printing plates or directly exposing PCB panels) is of the flat-bed type in which the sheet is disposed on a horizontal table for exposure by the light energy (e.g., UV light or infrared) produced by the scanner. Such scanning apparatuses are typically quite bulky because of the horizontal table. Also, such direct imaging systems expose one side at a time, and there are problems accurately aligning the two sides for double-sided exposure. Above referenced and incorporated herein by reference co-pending U.S. patent application Ser. No. 09/435,983 to Vernackt, et al., describes a LDI device that images two sides of a panel that is held vertically, and including relatively positioning the imaging beams of one side to the other.
Note that direct imaging in itself does not ensure proper alignment of the real PCB panel to be processed with other panels, especially with outer layers where the image has to match the drilled holes pattern. A linked registration-imaging engine may be used to ensure such proper alignment. In addition, automatic handling of PCB panels is desirable, and a modem LDI device may include such an automatic material handler. The manufacturing difficulties of precise alignment and handling described above are further amplified as the overall physical size of the PCB panel increases. A PCB panel can be up to 24 inches wide and up to 36 inches long (609.6 mmxc3x97914.4 mm). Even larger PCB panels are known to be used. An automatic material handling system for a LDI device that is described in U.S. patent application Ser. No. 09/511,646 to Vernackt (filed Feb. 22, 2000) entitled A SYSTEM, METHOD AND ARTICLE OF MANUFACTURE FOR DIRECT IMAGE PROCESSING OF PRINTED CIRCUIT BOARDS, and assigned to the assignee of the present invention.
Typically PCB panels to be direct imaged are coated with a photoresist material (photoresist). The photoresist can be any one of several materials well known in the art, for example Riston(copyright) Photoresist (E. I. du Pont de Nemours and Company, Research Triangle Park, N.C.) or Laminar(copyright) Photoresist (Morton Electronic Materials, Tustin, Calif.). In the industry it is believed that for a given photoresist, a given quantity of light energy E must be imparted to the photoresist to properly and completely expose or react the photoresist. This has been expressed in the form of a product of power of the light source and exposure time as expressed in Equation 1:
E=Ixc3x97txe2x80x83xe2x80x83Equation 1
Where:
I=intensity of the UV light (mW/cm2)
t=time of exposure (seconds)
E=energy (mJ/cm2)
1W=1 J/s
Direct imaging typically uses a laser as the source of exposing energy. Several types of lasers may be suitable as a laser light source for exposing photoresist in a direct imaging process. A commonly used laser is a continuous wave (CW) ultraviolet (UV) laser having a relatively low power of 1 to 4 watts. Such lasers are typically UV gas-ion lasers, and are available front, for example, Coherent, Inc., Santa Clara, Calif., and Spectra-Physics Lasers, Inc. Mountain View, Calif. Also solid state UV CW lasers are currently being developed. These also have relatively low laser power.
With the relatively low laser energy level that such lasers provide, extended exposure times are required to impart :he required level of power to the photoresist. This extended exposure time results in increased manufacturing time, among other shortfalls. Furthermore, due to one or more effects such as chemical migration of the exposed photoresist, using a relatively low power laser over an extended exposure time results in the formation of the inaccuracy-producing areas 214, 216 in the sides of the photoresist. The resulting PCB traces are then inaccurately shaped.
As geometries of the copper trace 220 become ever smaller, and the PCB panel becomes ever more crowded with traces and components, accuracy becomes extremely important.
As explained above, LDI has simplified the process of PCB exposure by eliminating the mask layer 206 and providing for increased accuracy in the manufacturing process. However, the lasers typically used still produce undesirable inaccuracies.
Furthermore, LDI devices typically use some modulation device to modulate the light pattern along a scan line. Such devices have a finite rise (and fall) time, so that the light beams cannot be turned on or off instantaneously. This too leads to loss of perpendicularity of sidewalls of exposed photoresist, with resulting resolution degradation and inaccuracies.
Thus there is a need for an method and apparatus to reduce the exposure time, increase the perpendicularity of the walls of exposed resist areas, increase the accuracy of placement of the sidewalls, increase the accuracy of the beginning and the end of an exposed photoresist line, and increase the accuracy of the resulting PCB trace.
A system, method and article of manufacture is disclosed that provides an improved, higher efficiency, more accurate laser direct imaging on a photosensitive medium on a substrate using a mode-locked laser having a low average power and a short pulse width. The mode-locked laser is scanned across the surface of the photosensitive medium. The resulting in-scan edges having improved perpendicularity relative to the underlying substrate.
Another embodiment, describes a method for laser direct imaging a pixel on a photosensitive medium with a laser beam. The method comprises providing a substrate having a first surface and an opposing second surface, and a photosensitive layer on the first surface. Then, emitting a mode locked laser beam, then receiving the laser beam by an acousto-optical modulator. Then receiving a modulating signal in the acousto-optical modulator and modulating the laser beam in the acousto-optical modulator. Next, emitting a modulated laser beam from the acousto-optical modulator to a first scanner unit. The scanner unit then receives the modulated laser beam and then directs the modulated laser beam across said first photosensitive layer in an in-scan direction to cause a first pixel of the first photosensitive layer to substantially photo-polymerize. The first pixel is defined by a surface area contacted by the modulated, pulsed laser beam and substantially penetrating through the first photosensitive layer to the first intermediate layer. The first pixel has a first side and a firstxe2x80x2 in the in-scan direction and a first beginning and a first end in the cross scan direction, the first side and the firstxe2x80x2 side being substantially perpendicular to the substrate.
Yet another alternative embodiment is a laser direct imaging apparatus for imaging a pixel on a photosensitive medium with a laser beam. The apparatus comprises a substrate, the substrate including a first surface and an opposing second surface and a first photosensitive layer on the first surface. The apparatus also comprises a mode locked laser operable to emit a pulsed laser beam and an acousto-optical modulator. The acousto-optical modulator includes a crystal oriented to receive the pulsed laser beam and a transducer. The transducer is in contact with the crystal and the transducer receives a modulating signal from an external source and then emits the modulating signal into the crystal to modulate the pulsed laser beam. The apparatus also includes a first scanner unit oriented to receive the modulated, pulsed laser beam. The first scanner unit directs the modulated, pulsed laser beam across the first photosensitive layer in an in-scan direction to cause a first pixel of the first photosensitive layer to substantially photo-polymerize. The first pixel is defined by a surface area contacted by the modulated, pulsed laser beam and substantially penetrating through the first photosensitive layer to the substrate. The first pixel has a first and firstxe2x80x2 sides in the in-scan direction and a first beginning and a first end in the cross scan direction. The first side and firstxe2x80x2 side are substantially perpendicular to the substrate.
In an alternative embodiment, the mode-locked laser is a ultraviolet (UV) laser, having a wave length of between 200 nm and 532 nm.
In an alternative embodiment, the mode-locked laser is modulated in a scophony mode. The resulting cross scan beginning and end of the scanned line has improved perpendicularity relative to the underlying substrate. The improved perpendicularity resulting in improved accuracy over the prior art.
In an alternative embodiment, the disclosed ultraviolet, mode-locked, scophony mode modulated laser is included in a dual side laser direct imaging system, method and apparatus wherein the substrate has a photosensitive medium on a first surface and on a second, opposing surface and the photosensitive medium on both sides of the substrate are substantially simultaneously imaged or scanned.
In an alternative embodiment the substrate is held in a frame which can be moved in a cross scan direction. The frame allows the substrate to be moved in a cross scan direction so that the ultraviolet, mode-locked, modulated laser can be caused to scan across the entire surface of the photosensitive medium.
The disclosed, improved, ultraviolet, mode-locked, scophony-mode-modulated laser imaging system, method and apparatus provides improved efficiency of up to a factor of three. This allows usage of a low, average power laser while still providing improved accuracy.